Climate Change Canada

 

4.2.1.3: Projected changes and uncertainties

Earth system models or global climate models provide projections of future climate change based on a range of future scenarios incorporating GHGs, aerosols, and land-use change (see Chapter 3, Section 3.3.1). The fifth phase of the Coupled Model Intercomparison Project (CMIP5, see Chapter 3, Box 3.1) was an internationally coordinated effort that produced a multi-model ensemble of climate projections. Results from this ensemble specific to Canada have been generated using output from 29 CMIP5 models, based on three scenarios: a low emission scenario (RCP2.6), a medium emission scenario (RCP4.5), and a high emission scenario (RCP8.5). Results for a fourth scenario that was part of the CMIP5 protocol (RCP6.0) are also available, but from fewer models. These multi-model results are described by Environment and Climate Change Canada (ECCC, 2016) and are available for download from the Canadian Climate Data and Scenarios website ().

In the following, multi-model climate change projections for 2031–2050 and 2081–2100 (relative to a 1986– 2005 reference period) are shown for Canada for a low emission scenario (RCP2.6) and a high emission scenario (RCP8.5), spanning the range of available scenarios. The low emission scenario assumes rapid and deep emission reductions and near-zero emissions this century, whereas the high emission scenario assumes continued growth in emissions this century. The two time periods were chosen to provide information for the near term (2031–2050), when differences in emission scenarios are modest, and for the late century (2081–2100), when climatic responses to the low and high emission scenarios will have diverged considerably. This latter difference illustrates the long-term climate benefit associated with aggressive mitigation efforts. The multi-model median change is shown in map form, along with time series of the average from individual models, which is computed for all Canadian land area. The box and whisker symbols at the right side of the time series provide an indication of the spread across models for 2081–2100. Values for different regions in Canada are provided in Table 4.2.

Projected temperature changes for winter (December–February average), summer (June–August average), and annual mean are shown in Figures 4.6, 4.7, and 4.8, respectively. Enhanced warming at higher latitudes is evident in the winter and annual mean. This is a robust feature of climate projections, both for Canada and the Earth, and is due to a combination of factors, including reductions in snow and ice (and thus a reduction in albedo) and increased heat transport from southern latitudes (see Chapter 3). This high-latitude amplification is not apparent in the summer maps because, over the Arctic Ocean, summer temperatures remain near 0ºC — the melting temperature of snow and sea ice. In the near term (2031–2050), the differences in pattern and magnitude between the low emission scenario (RCP2.6) and the high emission scenario (RCP8.5) are modest (on the order of 0.5ºC to 1ºC). However, for the late century (2081–2100), the differences become very large. Under the high emission scenario, projected temperature increases are roughly 4ºC higher, when averaged for Canada as a whole, than under the low emission scenario. The differences are even greater in northern Canada and the Arctic in winter. In southern Canada, projected winter temperature change is larger in the east than in the west, with British Columbia projected to warm slightly less than elsewhere in Canada. The projected summer change is more uniform across the country.

The maps in Figures 4.6, 4.7, and 4.8 illustrate the median projection from the CMIP5 multi-model ensemble — some models project larger changes and some project smaller changes. The spread across models provides an indication of the projection uncertainty discussed in Chapter 3, Section 3.3.2. The spread among the CMIP5 ensemble is only an ad hoc measure of uncertainty. Actual uncertainty could be larger, because CMIP5 models may not represent the full spectrum of plausible representations of all relevant physical processes (Kirtman et al., 2013). The spread across models also includes natural, year-to-year variability, which continues in the future much as it has in the past. Even when averaged for a region as large as Canada, differences in projected temperature among models are on the order of a couple of degrees. Under a low emission scenario (RCP2.6), annual mean warming in Canada stabilizes at about 1.8ºC above the 1986–2005 reference period after about 2050, whereas, under a high emission scenario (RCP8.5), annual warming continues throughout the 21st century and beyond, reaching about 6.3ºC above the reference period by 2100. Additional values for Canada as a whole and for various regions are presented in Table 4.2.

Temperature change is one of the key indicators of a changing climate, and many other climate variables are directly or indirectly tied to temperature. The changes in mean temperature are the projected response to emissions of GHGs and aerosols from human activities, and natural internal climate variability will continue to be superimposed on these forced changes. Natural internal climate variability is simulated by the climate models used to make projections of future climate change, and this is evident in the year-to-year variability in the Canada-average temperature time series in Figures 4.6, 4.7, and 4.8 (the individual thin lines). Indeed, this year-to-year variability looks much like what has been observed in the past (see Figure 4.2). In contrast, the underlying forced response (approximated by the multi-model average — the thick line in the figures) is a slowly, monotonically changing value that closely tracks the cumulative emissions of GHGs since the pre-industrial era (see Chapter 3, Section 3.4.1). In assessing the impacts of a warming climate, this combination of slow forced change and natural internal variability is important to keep in mind — the future will continue to have extreme warm and cold periods superimposed on a slow warming forced by human activities.

Because the components of the global climate system are closely interconnected, temperature change in a particular region, such as Canada, is closely related to the change in global mean. This is illustrated in the left panel of Figure 4.9, which shows Canadian mean temperature change versus global mean temperature change. As noted previously, Canadian mean temperature is projected to increase at roughly double the global mean rate, regardless of the forcing scenario. That is, the relationship between Canadian and global temperature change remains constant, as shown by the fact that the results from the different scenarios are all aligned. This connection between global mean and Canadian mean temperature change provides a way of estimating the implications of global change for Canada under alternative forcing scenarios. In other words, impacts estimated under one forcing scenario can be scaled to approximate impacts under another forcing scenario, since the ratio of Canadian to global temperature change is roughly constant. Of course, this assumes that impacts scale directly with temperature (which may not always be the case)

The IPCC Fifth Assessment concluded that “Global mean temperatures will continue to rise over the 21st century if GHG emissions continue unabated” (Collins et al., 2013, p. 1031). Because of the connection between global mean and Canadian mean temperature changes, it is virtually certain that temperature will also continue to increase in Canada as long as GHG emissions continue.

 

4.2.2: Temperature extremes and other indices

This subsection describes changes in temperature extremes and other indices relevant to impact assessments. All are derived from daily temperature data. Some indices, such as the annual highest and lowest day or night temperatures, represent temperature extremes and have widespread applications, such as in building design. Others are important for specific users. For example, degree days are a commonly used indicator of building cooling or heating demand, and of the amount of heat available for crop growth. Heating degree days (the annual sum of daily mean temperature below 18ºC) or cooling degree days (the annual sum of daily mean temperature above 18ºC) are used for energy utility planning, while growing degree days (the sum of daily mean temperature above 5ºC in a growing season) is an important index for agriculture. Some indices, such as the number of days when daily maximum temperature is above 30ºC or when daily minimum temperature is above 22ºC, have important health implications (Casati et al., 2013). Observed changes in temperature indices and extremes indicate that warm events are becoming more intense and more frequent, while cold events are becoming less intense and less frequent. These have important implications; for example, extreme winter cold days are important in limiting the occurrence of some forest pests (Goodsman et al., 2018).